The bromodomain is a characteristic motif of proteins found in transcriptional regulators. Proteins having a bromodomain usually contain one or two (Tamkun, J. W. et al., (1992), Nuc. Acids Res., 20:2603), but sometimes as many as five bromodomain motifs (Nicolas, R. H. and Goodwin, G. H. (1996), Gene, 175 (12):233–240). This motif is found in a wide variety of animals. For example, it is identified in yeast (Winston, F. et al., (1987), Genetics, 115:649–656; Laurent, B. C. et al., (1991), Proc. Natl. Acad. Sci. USA, 88:2687–2691), in Drosophila (Digan, M. E. et al., (1986), Dev. Biol., 114:161–169; Tamkun, J. W. et al., (1992), Cell, 68:561–572), and in the genes for transcriptional regulators in mammals (Denis, G. V. and Green, M. R. (1996), Genes and Devel., 10:261–271; Yang, X. J. et al., (1996), Nature, 382:319–324).
All transcriptional regulators containing a bromodomain serve to control signal-dependent transcription in actively proliferating cells (Tamkun, J. W. et al., (1992), Cell, 68:561–572; Haynes, S. R. et al., (1992), Nuc. Acids Res., 20:2603). Due to this feature of these transcriptional regulators, it is suggested that cancer may develop if the gene for the protein containing a bromodomain is not normally controlled. In fact, several studies have shown that human transcriptional regulators with a bromodomain RING3, p300/CBP, and PCAF may be involved in oncogenesis.
RING3 is a transcriptional regulator highly homologous with the fsh protein that regulates development of Drosophila (Haynes, S. R. et al., (1989), Dev. Biol., 134:246–257). RING3 is a nuclear serine/threonine kinase having autophosphorylating activity. This activity of RING3 correlates with a proliferating state in chronic or acute lymphocytic leukemia. For instance, when Denis and Green collected lymphocytes of peripheral blood from 10 patients suffering from leukemia, kinase activity associated with RING3 was identified in all of the 10 patients but not in normal controls (Denis, G. V. and Green, M. R. (1996), Genes and Develop., 10:261–271). Furthermore, this activity was not detected in the blood cells from patients whose leukemia had remitted by virtue of chemotherapy.
p300 and CBP (CREB binding protein) encode highly similar proteins and are thus often called p300/CBP. p300/CBP is a co-activator for a transcriptional regulator CREB (cAMP responsive element binding protein) (Kwok, RPS et al., (1994), Nature, 370:223–226), and is considered as a key protein for growth regulation. Mutation in p300/CBP has been found in familial or sporadic cancers. Germline mutation of CBP results in Rubinstein-Taybi syndrome, which causes patients to develop various malignant tumors (Petrij, F. et al., (1995), Nature, 376:348–51), while mutation in p300 is found in sporadic colorectal and gastric cancers (Muraoka, M. et al., (1996), Oncogene, 12:1565–1569). Furthermore, CBP is fused with MOZ (Monocytic leukemia Zinc finger protein) in a t (8; 16) (p11; p13) translocation found in a certain kinds of acute myelocytic leukemia. The fusion protein has histone-acetyltransferase domains derived from both genes (Bannister, A. J. and Kouzarides, T. (1996), Nature, 384:641–643; Orgyzco, V. V. et al., (1996), Cell, 87:953–959; Brownwell, J. E. and Allis, C. D. (1996), Curr. Opin. Genet. Devel., 6:176–184). Since acetylated histone is known to be associated with transcriptionally active chromatin, the fusion protein may be involved in leukemogenesis by way of aberrant histone acetylation (Brownwell, J. E. and Allis, C. D. (1996), Curr. Opin. Genet. Devel., 6:176–184).
p300/CBP is also considered to be associated with cancer since it interacts with known oncogene products. For example, p300/CBP binds to E1A protein (Arany, Z. et al., (1995), Nature, 374:81–84), one of the early genes of adenovirus. p300 is also a co-activator for transcription factors, c-Myb (Dai, P. et al., (1996), Genes Dev., 10:528–540) and c-Fos (Bannister, A. J. and Kouzarides, T. (1996), Nature, 384:641–643).
PCAF, is considered to inhibit the interaction of E1A with p300/CBP by competing with E1A for binding to p300/CBP (Yang, X. J. et al., (1996), Nature, 382:319–324). PCAF also has histone-acetyltransferase activity.
Thus, it is thought that transcriptional regulators containing a bromodomain are involved in regulation of cell growth, and that their aberrant regulation may be closely related to various diseases, particularly to cancer. Transcriptional regulators containing a bromodomain have thus recently received much attention as novel targets for specifically treating cancer.